Machining Flanges

Flanges connect two things and keep what is inside from getting out. A pump flange, pipe flange, gearbox flange, or custom mounting flange has to bolt up square, seal without leaking, and share load across the bolt circle. This guide covers how flanges are machined, the facing and bolt-circle work that defines them, sealing-surface finishes, and what drives the cost.

The Three Features That Define a Flange

Every flange comes down to three machined features that must relate to each other:

• The face, which must be flat and, on a mating pair, parallel and square to the bore or pilot
• The bolt circle, the ring of holes on a bolt circle diameter (BCD) that must land on-position so the flange lines up with its mate
• The sealing surface, a raised face, groove, or O-ring land that carries the gasket or seal

Miss any one and the joint leaks or won't bolt together. That is why facing flat, bolt-circle position, and sealing-surface finish are the headline requirements on almost every flange print.

Step 1: Facing the Flange Flat

Flange facing is the operation that makes the mating surface flat and true. On round flanges we face on a CNC turning lathe, which produces a naturally flat, square face referenced to the bore in one chucking. On square or irregular flanges we face on a CNC milling machining center.

• Flatness: .001 in to .003 in across the face is typical, .0005 in for high-pressure seals
• Perpendicularity of the face to the bore or pilot: .001 in to .005 in depending on class
• Parallelism of the two faces on a spacer or blind flange: .002 in typical

Facing both faces and the bore in a single setup is what keeps a flange square. Every re-clamp adds error, so we transfer to a sub-spindle or fixture the part off a machined datum rather than re-gripping raw stock.

Step 2: The Sealing Surface

The sealing surface is where finish matters most. The gasket or O-ring needs a controlled roughness to grip and seal. Common styles and targets:

• Raised face (RF) with a serrated or phonographic finish: a controlled spiral groove, commonly Ra 125 to 250 uin, which lets a soft gasket bite in
• Flat face (FF): similar controlled finish, used with full-face gaskets
• Ring-type joint (RTJ) groove: machined to hold a metal ring, finish Ra 63 uin or better on the seating surfaces
• O-ring land or face groove: smooth finish Ra 16 to 32 uin so the elastomer is not cut, with groove width and depth held to +/-.002 in

Read the print carefully on sealing surfaces. A pipe flange often wants a rough, controlled serration so a fiber gasket grips, while an O-ring land wants a smooth surface. The two requirements are opposite, so the callout, not habit, sets the finish.

Step 3: Drilling the Bolt Circle

The bolt circle is the pattern of holes on a bolt circle diameter. Position is what makes a flange interchangeable with its mate. We drill and, when needed, ream bolt holes on a machining center with the part located off the bore or pilot, so the pattern is true to the flange centerline.

• Hole position: +/-.005 in to +/-.010 in on clearance holes, tighter when holes are reamed for fitted bolts or dowels
• Bolt circle diameter: held to +/-.005 in typical
• Straddle the centerline: most standard flanges are drilled so no bolt hole sits on the vertical or horizontal centerline, which lets the flange mate in more than one orientation

For custom flanges we match the mating pattern from the print or a sample. Counterbores, spotfaces, and tapped holes get done in the same setup as the drilling so the whole pattern stays coordinated.

Materials for Flanges

• A105 forged carbon steel and A350 LF2 for standard pipe flanges
• 304/316 stainless for corrosion and sanitary service
• 4140 alloy steel for structural and drive flanges
• Aluminum 6061-T6 for lightweight mounting and vacuum flanges
• Ductile iron for large, cast pump and valve flanges

Flanges are commonly machined from plate, forgings, or castings rather than round bar. On defense and controlled work we run DFARS-compliant material and pull certs traceable to the heat. Our government and defense machining page covers traceability and packaging.

Inspection

We verify flatness, bolt-circle position, and sealing-groove dimensions on a Brown & Sharpe CMM, which measures true position of every hole against the bore datum in one report. Sealing-surface finish is checked with a profilometer against the print callout. New part numbers get a first article inspection with a full dimensional report, and controlled jobs ship with material certs. The inspection and quality process is ISO 9001-aligned.

Flange Cost Drivers

The table shows typical illustrative ranges for a machined carbon steel flange roughly 6 in OD with an 8-hole bolt circle and a raised face, from plate, in a lot of 25. These are illustrative estimates, not a binding quote.

To control flange cost, use a standard bolt pattern where possible, hold clearance-hole tolerance rather than reaming unless fitted bolts are required, and match the sealing finish to the actual seal. Send your flange drawing to our instant quote tool for a fast estimate.

Typical Lead Times

Standard machined flanges from plate run 1 to 2 weeks. Flanges that need forgings or castings brought in, or added coating and heat treat, run 3 to 4 weeks.

Summary

A leak-free, bolt-up-square flange is built on three controlled features: a flat, square face, a true-position bolt circle referenced to the bore, and a sealing surface finished to match its gasket or O-ring. Machine the face and bore in one setup, drill the bolt circle off the bore datum, finish the seal to the callout, and verify true position on a CMM. Do that and the joint seals the first time.